The  test  results  with   electron  backscattered   difraction(EBSD)and   scanning   electron  microscope(SEM)  study  and  mechanical  properties  examination  show  that  the  structure  of microalloying  X70

X80  and  X100  pipeline  steels consists  of  acicular  ferrite

granular  bainite  and  a  little  lower  bainite

and  with  decreasing  effective  grain  size

increasing bainite content and improve structural homogeneity the strength and toughness of high grade pipeline steels increase obviously.